Fix crash at manual init

[apozdeyev]

In our project, we faced with a crash similar to https://github.com/bfeher/BFPaperTabBarController/issues/3. It was because tabRects array was set by empty array at tabbar vc initialization and later it will not be recreated when new view controllers are set. So it needs just set it to nil when new view controllers are set and it will be recreated at next accessing.